Containers or Dewars for maintaining nitrogen in a liquid, cryogenic condition are usually formed as a metal can in which the liquid nitrogen is loaded; the can is usually surrounded by a shell. The volume between the can and shell is evacuated to decrease the heat conduction from outside of the shell to the interior of the can. In addition, it is usually desired to minimize the transmission of radiant energy from outside of the shell to the exterior surface of the can.
In the prior art, relatively low amounts of radiation have been transmitted between the shell and exterior of the can by forming both the can and the shell of an aluminum alloy having a very high percentage, such as 99%, of aluminum. The interior of the shell and exterior of the can are usually mechanically polished to a lustrious high-gloss, an operation that also substantially removes tool marks applied to the shell and can during the machining of these parts. After the can and shell have been mechanically polished, they are vapor degreased to remove filings, dirt and other foreign materials from the can and shell surfaces so these surfaces have relatively low radiant energy emissivities of approximately 0.024 at the temperature of liquid nitrogen, 77K. Radiant energy emissivity is defined in the usual manner, i.e., as the ratio of radiation emitted by a surface to the radiation emitted by a perfect black body radiator at the same temperature.
While the prior art techniques for reducing the emissivity of the shell and can are satisfactory for many purposes, the emissivity was not sufficiently reduced for other purposes. In particular, if it is desired to maintain the nitrogen in a liquid state for a prolonged duration, such as three months, the emissivity of the prior art can and shell are excessively high if the aluminum is only polished.